The slowest spinning X-ray pulsar in an extragalactic globular cluster

TL;DR

This paper reports the discovery of the slowest known X-ray pulsar in an extragalactic globular cluster, providing new insights into neutron star spin-up processes and binary formation in dense stellar environments.

Contribution

It presents the first detection of a 1.20 s pulsar in a globular cluster outside our galaxy, challenging existing models of pulsar recycling and spin evolution.

Findings

The pulsar is persistent over 12 years of observation.

It is in a wide binary with a 30.5-hour orbital period.

The neutron star has recently started accreting from a companion.

Abstract

Neutron stars are thought to be born rapidly rotating and then exhibit a phase of a rotation-powered pulsations as they slow down to 1-10 s periods. The significant population of millisecond pulsars observed in our Galaxy is explained by the recycling concept: during an epoch of accretion from a donor star in a binary system, the neutron star is spun up to millisecond periods. However, only a few pulsars are observed during this recycling process, with relatively high rotational frequencies. Here we report the detection of an X-ray pulsar with $P_{\rm spin} = 1.20$ s in the globular cluster B091D in the Andromeda galaxy, the slowest pulsar ever found in a globular cluster. This bright (up-to 30% of the Eddington luminosity) spinning-up pulsar, persistent over the 12 years of observations, must have started accreting less than 1 Myr ago and has not yet had time to accelerate to hundreds of Hz. The neutron star in this unique wide binary with an orbital period $P_{\rm orb} = 30.5$ h in a 12 Gyr old, metal rich star cluster, accretes from a low mass, slightly evolved post-main sequence companion. We argue that we are witnessing a binary formed at relatively recent epoch by getting a $\sim 0.8M_\odot$ star in a dynamical interaction -- a viable scenario in a massive dense globular cluster like B091D with high global and specific stellar encounter rates. This intensively accreting non-recycled X-ray pulsar provides therefore a long-sought missing piece in the standard pulsar recycling picture.